Introduction to depth of field

In addition to the complicated aspects of exposure and photometry, photographers usually find that depth of field is the most difficult concept to master. I realize the practical value of depth of field in solving practical problems and obtaining creative special effects. The following is the basic content of depth of field I have compiled for you for your reference, hoping to help friends in need.

Understand the basic theory

Generally speaking, the depth of field is an "acceptable clear area" that extends before and after adjusting the focal length. In fact, in any photo, only the plane in focus is really clear. However, in the observer's view, the objects before and after this plane may also appear quite clear. The difference of clear distance is based on several standards, and we will touch on the following parts. Teaching materials often discuss the "minimum dispersion circle" when explaining the depth of field, but that is a technical problem and there is no need for practical application. For example, in the nature zoo, you focus on the eyes of the Bengal tiger in the shadow, and its eyes are the clearest on the negative. At this time, the tiger's mouth and the bark behind it also showed an acceptable and clear image in the final photo. When you look away from your focused eyes, the degree of blur gradually increases. The farther away from the tiger's head, the worse the clarity of objects in the near foreground and far background.

Select the "appropriate" aperture.

Aperture diameter is the basic factor affecting depth of field. Generally speaking, a small aperture (represented by a large f/ value), such as f/ 16 or f/22, will produce a wide clear focus range. On the contrary, a large daylight aperture (represented by a small f/ value), such as f/2.8 or f/4, produces a shorter depth of field and a much smaller acceptable clear range in the foreground and background.

To create a work instead of simply taking a photo, the choice of aperture is a basic element. Even when using the program exposure mode, if possible, you should choose the most suitable combination of aperture and speed. However, it should be noted that because of the hand-held camera shooting, if it is exposed for a long time, the movement of the subject or camera shake may blur the photo. Therefore. You have to compromise: choose a larger aperture that is far from ideal to keep enough shutter speed.

For example, shoot a scene of a frame formed by a stone arcade in Britain. You can focus on the fountain near the middle lens and press the shutter. Six shots were actually taken, the first shot was taken with f/4, the second shot with f/5.6, and so on until it was taken with f/22. After that, you can carefully observe the printed 8X 1O inch photos.

The arcade and the background castle in the first photo taken with f/4 are not clear. All this is taken with f/ 1 1. It looks "almost clear" but it still doesn't meet your standards. The images of arcades, fountains and background castles taken by f/ 16 are clear enough, and your friends think this is the best photo. However, the image taken by f/22 is blurred, which is caused by the use of low-speed shutter, no tripod and camera shake.

Photographs taken with a telescope often have a depth of field, which makes some people think that this is an inherent optical property of telephoto lenses. In fact, as explained in the article, this statement is debatable. (Tuli AF400 mm f/5.6AT-X lens with F/5.6 and Kodak Elite 100 film. )

Choose a longer or shorter focal length.

Anyone who uses 28 mm and 300 mm focal length lenses will find that wide-angle photos usually have a wide range of depth of field. On the contrary, those photos taken with long focal length are generally shallow in depth of field. This situation usually leads to a simple conclusion: under any known aperture block, the longer the focal length, the shorter the depth of field. If you need a photo with a larger depth of field, shoot it with a shorter focal length.

The above seems to be the evaluation of depth of field in practice. But in fact, these statements are quite confusing. It is an optical fact that the depth of field will not change with different focal lengths. Don't you believe it? Then use 35 mm lens and 200 mm lens for specific test. First, take a long shot of a friend standing by a fence full of flowers. Then put on a wide-angle lens and shoot it at close range, so that she can be as big as the previous image in the viewfinder. Look at these slides with a magnifying glass of 10 times, and you will find that the acceptable clear focus range (check those flowers) is similar. Suppose they have different perspectives: the expressions of lines and shapes are definitely inconsistent, and the surface distances of objects at various distances are also inconsistent. Wide-angle lens produces "enlarged viewing angle" and increases the surface distance between objects. This can create an optical illusion that careless observers will think that the depth of field in these photos is different.

Only when used at the same shooting distance and the same aperture, the wide-angle lens will produce a greater depth of field than the telescope lens. However, due to the different depth of field on the surface, the choice of focal length and shooting distance has become an important creative consideration. (Longteng AF2O-4O mm zoom lens, f/ 16, with polarizer and Vervia film. )

Change the focal length used in the same position.

To be fair, telephoto lenses do reduce the depth of field, while wide-angle lenses enlarge the depth of field-but only if you shoot at the same distance. In fact, no matter what lens is used, the size of the image will affect the depth of field. Whether you take a full picture with a long focal length lens from a distance or a short focal length lens before you walk to the main body, the depth of field will be shallow.

For example, sitting on the grass opposite a historic building, shooting its sunshine with a 28 mm lens at f/5.6. It's still in the same position, but it's changed to a zoom lens of 100-300mm, and three shots are taken, with focal lengths of 100mm, 200mm and 300mm respectively, all of which are f/5.6. Looking at the photos, you will find that the doors of the buildings in these photos are getting bigger and bigger, but the acceptable focus range is obviously reduced with the extension of each focal length.

So, can a long focal length lens really produce a smaller depth of field than a wide-angle lens? Yes, but this can only be used at the same shooting distance. This is because the magnification of the subject image increases, thus reducing the acceptable focus range.

Change the distance between the camera and the subject.

As I mentioned, the depth of field is affected by the distance from the camera to the subject. When you focus on something very close to the lens, the depth of field is very small. When you focus at a long distance, the depth of field will be larger, and the depth of field will change with the focal length.

Tip: When taking close-up photos, such as macro photography, please remember that the depth of field will be in inches. The camera should be placed so that its film plane is parallel to the plane of the subject-perhaps the wings of the monarch butterfly.

You don't have to use a small aperture like f/32 to keep the butterfly completely within the depth of field. With a larger aperture, a higher shutter speed can be used, which will reduce the risk of image blurring caused by camera or subject movement. Another advantage is that most lenses can provide higher resolution when an intermediate aperture is used.

For example, if you use a 30 mm lens and the nearest focus distance is 8 feet, suppose you choose the aperture f/8. You found a flowering cactus in the Sonora desert. You took some photos in the car. At this time, you still feel too far away, in the infinite position of the lens ruler. After that, you go forward and take a second photo until you are 8 feet away from the subject.

1The cactus in the beautiful scenery in the photo is quite clear, at least it can be recognized; The cactus in the background also looks very clear. However, in recent photos, only your favorite cactus is clear. The cactus in the background becomes a light green block with no outline, while the cactus in the foreground is blurred by the shallow depth of field. In both cases, the farther the cactus is from the focus, the less clear it is.

Remember: when you are close to or away from the subject, the other two factors-the size and perspective of the subject in the frame will also change. For example. The shape and line appearance at 10O feet and the surface distance between objects are quite different from those at 8 feet (this can be seen in the viewfinder. )

Determine the "correct" focus

As common sense, the depth of field has the following distribution modes in the usual shooting distance: the depth of field extends about 1/3 to the front of the focus and about 2/3 to the back of the focus. Do you want a large depth of field? Then set the focus roughly at 1/3 of the big scene.

Especially when shooting with an autofocus single-lens reflex camera, it is easy for the camera to set the focus. Although "hasty shooting" may be necessary to capture fleeting dynamic moments, it is also important to control the accurate focus if the environment permits. Even for an autofocus camera, this is very simple-usually, you can lock the focus by simply pressing the shutter or re-composing.

For example, at the racing scene, you see three high-performance sports cars parked nearby, with a black Porsche in the foreground, a red BMW in the back and a white lotus in the background. At this time, you find a celebrity leaning against the nearest car, and his blue eyes are clearly visible. You must make a decision immediately: focus on the BMW car near the middle scene to get the maximum depth of field and make the three cars clear at the same time; Or carefully focus on the person in the foreground, so that you can take a complete picture of him and show it to your friends?

When you take a clear target in the scene as the main shooting object, you should usually focus on the most important place: the eyes of the characters, the letters on the side of the yacht, or the paintings on the cave walls. Although you mentioned the control of depth of field in other parts, depth of field has become a secondary issue at this time.

Select hyperfocal distance focus.

There is a special point that can produce the maximum depth of field-a focal point is precisely aimed at this "hyperfocal distance" position. Being the closest focus can still keep the receiver clear enough at infinity, so that the depth of field extends from half of the focus distance to infinity. Of course, due to the different aperture and focal length of the lens, the depth of field will be different. However, this technology does increase the depth of field of your photos, and you don't have to minimize the aperture, so the image will be blurred because of camera vibration or subject movement.

For example, you shoot Times Square in the middle of the road with a 50 mm lens. You want to shoot the whole area clearly, but you can't reduce the aperture to more than f/ 1 1 (even a smaller aperture requires a slow shutter of 1/30 seconds, which will blur the moving car). With hyperfocal distance's method, you can set the focal length to 30 feet, which replaces the autofocus system focusing on the blonde in the foreground. On the viewfinder screen, most images seem to be out of focus, which makes them look unpleasant. You insisted on taking this photo, and found that all the scenes from 15 feet to infinity in the printed photo were quite clear, that is, they were all within the depth of field.

Use less infinite focus.

I seldom use infinite focus when shooting fields and city scenery, because in this way, any foreground part may be blurred, and such subjects often play an important role in the picture-first of all, they can add the effect of three young people to the photos of two young people. Furthermore, with this method, most of the depth of field will lose its practical value to the picture after infinity, and it will "waste" the depth of field.

However, when shooting sports, racing cars and wild animals, it may be useful to focus at infinity if the subject is far away. In this case, you can tolerate some blurred images in the viewfinder-first, because your main purpose is to shoot absolutely clear exposed objects.

Predictive depth of field

Some single-lens reflex cameras have a depth of field prediction button, which can reduce the lens aperture to the selected actual "working" aperture. This is necessary for visual depth of field, because without it, you can always see the scenery only through the maximum aperture of modern cameras.

Press the depth of field prediction button, you can see the scene effect of f/8, f/ 1 1 or f/22 respectively, and you can evaluate which part of the scene is very clear, generally clear or completely blank at a glance. The focus screen will naturally darken, almost darkening at the minimum aperture, unless the illumination is extremely complete. Before you decide that this control system is "useless", please remember that over time, your eyes will adapt to dim light.

Adjust the aperture to f/4, then to f/5.6, and so on, and slowly move to a smaller aperture. At this time, because the viewfinder screen darkens one after another, your eyes will open wider to improve your vision in low light. Even so, on cloudy days, if you shoot with f/22, depth of field prediction may be really useless. The observation screen is too dark to evaluate the acceptable clear area. When taking close-up photos of nature, you can use a flashlight to illuminate the subject as a focusing aid. In general, if the aperture used is at least not less than f/ 1 1, such preventive measures are unnecessary.

But remember, any visual measurement of depth of field is an estimate, not a technology. For example, a foreground that looks clear on a small viewfinder will appear blurred in a photo of 16×20 inches. However, especially in high-magnification photography (using macro or telescope), depth of field prediction is a very valuable device. Especially in the case that the acceptable clear range is very shallow and short, it is very important to make the deepest prediction at several apertures and focus, which is helpful to create technically excellent images.

For example, when you take an environmental portrait of a blacksmith in a village, you use an 80-200mm f/2.8 zoom lens to shoot at the 200mm end, you set the aperture to f/8 to get the correct exposure, and you shoot quickly to capture the moment when the hammer hits the nail. When your photo is developed, the outline of the green off-road vehicle in the background can be recognized immediately, and a partially blurred baseball cap is inserted in the foreground. When you take pictures with f/2.8, you can't see either, but in the images taken with f/8, both become serious distractions.

In order to avoid such mistakes, especially when shooting with binoculars, the deepest prediction system should be used. Depth of field prediction is useful-unless you shoot with the maximum aperture of the lens, only in this way can you say that no prediction of depth of field is accurate.

Check the lens, Jing Shenbiao.

To be honest, some photographers admit that it is very difficult to visually observe the depth of field, while others don't think this technology is quite reliable or accurate at all. If you belong to one of these categories, learn to use this logo on the camera to identify Jing Shenbiao. At present, many people don't study, while others find that the Jing Shenbiao on some zoom lenses is poorly designed or poorly designed: it is often difficult (or impossible) to judge the depth of field of any known focal length/aperture combination.

If your lens has the right proportion, focus first, and then decide the aperture that can provide enough depth of field. Now look at the two f/ values of the selected aperture on the depth of field table, which represent the acceptable clear distance (near and far) range in the final image. If necessary, you can change the focus distance. To increase the depth of field, you can also reduce the aperture you may need to use by one level.

For example, you are lying on sunny Cape Cod beach and see your daughter building a sand castle 0/0 feet away from the waves/kloc. There is a ship with colorful sails approaching along the coastline on the sea. You focus on the sandcastle and adjust the aperture of the 50 mm lens to f/ 16 to get enough depth of field. At this time, Jing Shenbiao shows that everything from 7 feet to 2 1 foot will be clearly displayed.

Unfortunately, the ship is at least 100 feet away, so it will be a little fuzzy. But no problem, you just need to turn the focus ring so that the "16" mark (f/ 16) on the left side of the depth of field scale center is aimed at the infinity symbol. Now look at the "16" mark on the right. You will see it is 8 feet away from the scale. As an extra insurance measure, adjust the focal length to 8 feet and reduce the aperture to f/22. In the final photo, the child will be very clear and the ship should be quite clear, unless a slow shutter is used to blur the moving body.

Do this exercise with the camera in your hand until you feel comfortable with it. Then, this technique is useful for any lens with appropriate marks to accurately control the depth of field in any case.

In the 4 X 6-inch photo, due to the large depth of field, both buildings seem to be in focus; In the 8 X 12 inch photo, the buildings in the distance are not so clear, but they are still acceptable; But when the photo is enlarged to 16 X 2O inch, the lack of clarity becomes very obvious. (Canon EF20-35mm zoom lens, 20m end shooting, f/ 1 1, Fuji super G2OO film. )

Consider other factors that affect the depth of field.

First of all, the depth of field depends on subjective judgment. You may think that the image is "still clear" if it is blurred to a certain extent, while more critical people may think it is "blurred".

Serious macro photographers should know that the depth of field is evenly distributed in high-magnification close-up works. The depth of field before and after the focus plane is approximately equal.

The clarity we can see also depends on the projection distance of the slide, the size of the photo and the distance to observe the photo. The larger the photo, the smaller the acceptable clear range. Foreground objects that appear to be in focus on a 4×6-inch enlarged photo will appear blurred on a photo enlarged to 16×20 inch or a photo viewed near 8× 10 inch.

All distances mentioned in this paper are related to the use of 35 mm photographic equipment. The concept of using a large negative is similar, but under the same aperture and the same shooting distance, the depth of field will actually decrease. This is because a larger frame requires a longer focal length to fill the viewfinder from the same shooting position. For example, a 105mm lens on a 35mm camera is equivalent to a 200mm lens on a 120 camera or a 36omm lens on a 4X5 camera. In the same shooting position, using a longer lens to obtain the same image frame will produce a shallow and acceptable clear area.

This is one reason why some landscape photos taken with 4X5 cameras are taken with f/32 or even f/64. However, the rocking and pitching functions of the landline can be used to increase the depth of field of the large aperture. This performance can keep the film plane parallel to the subject, because the lens can leave the optical axis. Some 35mm cameras can also use this technology for lenses with perspective correction and pitch/swing functions.

In some public materials, including the lens instructions, you can find the complete depth of field map besides the hyperfocal distance map. These materials allow photographers to quickly determine the depth of field at different focusing distances, with any aperture and a specific focal length.

When you put all this information together, you can wisely decide how to get the correct depth of field in any particular situation. This depth of field may be small in a portrait-one is to blur the messy or secondary elements that distract the audience. Or in a classic landscape work, the depth of field will be wide-to keep the fence in the foreground, the horse in the middle and the snow-capped mountains in the distance clear. Repeat the experiment, take notes, and then compare the photos to make sure you are doing it right.

Then, read some professional technical books. Familiar with the "minimum dispersion circle", the perspective difference of various shooting distances and other "complex things". If there is only one reason: the ability to control the depth of field can distinguish snapshots from advanced photographers, then these extra efforts are worth it.